308 G. H. WILLIAMS — STRUCTURE OF THE PIEDMONT PLATEAU. 



ever, quite confidently assert the sedimentary origin of certain gneisses and 

 schists among the eastern Maryland crystallines as follows : 

 «. Typical biotite gneiss or biotite-muscovUe gneiss, highly feldspathic and 

 containing thin parallel layers of varying composition, indicative of 

 original bedding. These are now crystallized as biotitic, hornblendic, 

 or epidotic schists. These gneisses are well exhibited at the great 

 quarries which have been opened in them on Jones and Gwynn's 

 falls, near Baltimore. They are cut by numerous dikes of coarse- 

 grained muscovite-pegmatite, or biotite-pegmatite,* and are filled with 

 an abundance of quartz or pegmatite eyes and lenses, intercalated 

 parallel to their bedding. Similar rocks are also exposed in the 

 vicinity of Washington, where well characterized beds of conglomerate 

 have been observed in them. 



b. Muscovite gneiss. This rock is composed mainly of muscovite and 



quartz, with comparatively little feldspar, but it is full of character- 

 istic contact minerals, such as garnet, staurolite, cyanite, fibrolite, 

 rutile, etc. This grades into a typical — 



c. Mica schist, devoid of feldspar, but containing the same contact minerals. 



This is usually of very limited extent in the eastern area, and is to be 



regarded as a limited facies of the mica gneiss. Both of these rocks 



are full, as are the sericite schists in the western area, of veins of white 



quartz, colloquially known as flint. 



Quartzite. — The highly siliceous rocks of the eastern area, though in all 



probability of sedimentary origin, have lost all traces of clastic structure 



through metamorphism, unless they were conglomeratic. The nearly pure 



quartz rocks of the eastern area show a much greater variety than those of 



the western, as will be seen from an enumeration of the following distinct 



types : 



a. Quartzite or Quartz-schist. This is best exhibited in the abrupt east-west 

 hill, north of Baltimore, known as Setter's (Tyson : " Sater's ") ridge- 

 It is here extensively quarried, and is such a clearly marked rock type 

 that it serves at many other localities to fix a definite horizon. The 

 rock is a completely crystalline mosaic of quartz grains which inter- 

 lock by complicated sutures. It contains a small proportion of ac- 

 cessory minerals and no indication of original clastic structure. Its 

 structure is illustrated in Mr. Keyes' figure 5 (page 321). There is 

 always present a perfect foliation, due to parallel layers of muscovite 

 at varying distances from each other. In these foliation planes there 

 is an abundant development of black tourmaline, whose crystals are 

 always transversely broken, and their fragments more or less sepa- 

 rated, as if by stretching. 



* See Johns Hopkins University Circulars, no. 38, vol. 4, 1885, p. 65. 



